A variety of types of car jack have been developed over a long time for facilitating a car driver to replace a flat/punctured tire or do necessary maintenance to the car. Among these types of car jack, a so-called "frame-type" car jack is found specially advantageous to carry in a car since it is very compact under a folded state. According to the mechanism for generating the lifting force for the car jack, the "frame-type" car jack can be classified into two kinds: one is a hydraulic frame-type car jack and the other is a threaded frame-type car jack wherein the former can be manually or electrically operated and the latter usually is manually operated.
FIG. 1 shows a conventional frame-type, manually-operated hydraulic car jack 90 which comprises a base 10, a pressure release valve 16, a socket 14, a lever 18, a hydraulic oil cylinder 12, a piston 122, a pair of pulling bars 20, a pair of links 22, a pair of pushing bars 24, and a load-engaging head 26. When an end of the lever 18 is inserted into the socket 14 and has a well-known reciprocating movement relative thereto, due to a hydraulic force, the piston 122 will be lifted. The lift of the piston 122 causes the load-engaging head 26 to also have an upward movement, via the transmission of the pulling bars 20, the links 22, and the pushing bars 24, which are sequentially pivotably connected together and form a frame of the car jack 90 wherein the load-engaging head 26 is pivotably connected to a top end of the pushing bars 24, the links 22 have an end pivotably connected to the base 10 and the pulling bars 20 are pivotably connected to the piston 122.
In addition to the above mentioned manually-operated hydraulic system, the piston 122 can also be lifted by an electrically-operated hydraulic system.
Furthermore, it is obvious to those skilled in the art that in a conventional threaded frame-type car jack, the piston 122 is replaced by a threaded shaft which is lifted or lowered by a rotating action.
However, all of the three types of conventional frame-type car jacks have a common disadvantage that they cannot prevent an unintentional lowering thereof when they are supporting a lifted heavy load, for example, a car. This is particularly true for the hydraulic frame-type car jack since a hydraulic driving mechanism is more unreliable than a threaded driving mechanism although the former is more convenient and easier to operate.
An unintentional lowering of the car jack is very dangerous to the driver who may lift front or rear wheels of the car and crawl into a underside of the car to carry out maintenance or repairs.
Thus, many conventional car jacks having operating manuals clearly require that the driver, when using a car jack like the car jack 90 to lift front wheels of a car 95 as indicated by phantom lines of FIG. 2 of the present application, should also use a fixed supporting stand 91 to support the car 95, thereby to avoid the danger caused by an unintentional lowering of the car jack 90. However, in fact, most drivers will not spend extra money to buy the fixed supporting stand 91 when they are purchasing the car jack 90. Rather, in many cases, they prefer to use a stack of bricks to support the lifted car. Such a support by a stack of bricks is not secure and sometimes may cause a catastrophe.